Siren-horn circuitry

ABSTRACT

Variable audiofrequency signals for use in alarms, entertainment devices and like applications requiring sonic outputs from rugged and economical miniaturized equipments which consume little power, are provided by a transistorized circuit wherein sinusoidal outputs from a simple phase-shift oscillator govern the frequencies at which a voltage-controlled audio oscillator drives its load in the form of a voice coil of an inexpensive loudspeaker.

United States Patent [72] Inventor NormanS.Goralnick 233 Worthen Road, Lexington, Mass.

m m n .m u w M mm m an m CH .w 904 67 99 a ll n l/- 42 m m 86 46 E 03 49 33 P. %7 n nm u m 08M& q d N m L n mm fla AFP HT 224 Attorney-James E. Mrose ABSTRACT: Variable audiofrequency signals for use in alanns, entertainment devices and like applications requiring sonic outputs from rugged and economical miniaturized equipments which consume little power, are provided by a transistorized circuit wherein sinusoidal outputs from a simple phase-shift oscillator govern the frequencies at which a voltage-controlled audio oscillator drives its load in the form of a voice coil of an inexpensive loudspeaker.

E mum b3 8/ w 3 PAT-ENTEUSEP nan INVENTOR NORMAN S. GORALNICK ATTORNEYS SIREN-HORN CIRCUITRY BACKGROL JND OF THE INVENTION The present invention relates to improvements in electronic generation of variable-frequency tones, and, in one particular aspect, to novel and improved miniature electronic sound generators in which unique transistorized circuitry minimizes the electrical power and the number and costs of components required to produce siren-type audio signals.

Numerous electromechanical and electronic versions of sound-producing horns, sirens and the like have been devised in the past in efforts to take advantage of the versatile simulating capabilities of electrical equipment. For some purposes, such as the provision of both horn and siren sounds for models and amusement vehicles, it becomes highly important that such devices operate efficiently with low power drain from a conventional miniature power source, and that it be possible to fabricate reliable and sturdy units at particularly low cost from as few and as noncritical and inexpensive components as possible. in the latter connection, significant economies, efficiencies and simplifications are realized through practice of certain of the present teachings having to do with the control of a two-transistor audio oscillator by the sinusoidal output voltages from a single-transistor phase-shift oscillator, the audio oscillator including a speaker voice coil as a wellmatched component. By way of distinction, prior approaches have required use of matching transformers in association with loudspeakers, and have relied upon more costly, critical and complex multivibrator networks.

SUMMARY The present invention is aimed at creating an improved and reliable horn-siren electronic network which lends itself to fabrication in miniature proportions and involves only relatively few noncritical low-cost components. A preferred embodiment utilizes three inexpensive germanium transistors, one of which functions in a first-stage phase-shift oscillator which delivers a low frequency substantially sinusoidal output voltage to a special form of second-stage voltage-controlled oscillator which includes the remaining two transistors. The latter oscillator involves capacitively-coupled feedback which sustains periodic electrical pulse flows through one of its transistors, the pulse repetition rate being in the audiofrequency range and being modulated in repetition rate by the varying voltage delivered by the first-stage phase-shift oscillator. Advantageously, the low-impedance voice coil of a speaker is coupled serially in the current-flow path through the said one of the second-stage transistors, providing a good-quality impedance match directly, and responding to the frequencymodulated current pulses by developing related sonic outputs in the audiofrequency range. Attention-arousing siren characteristics are produced in the described operating mode, despite the obvious lack of sinusoidal characteristics in the pulse signals, and an unvarying hornlike tone, also rich in harmonies, is readily achieved through switched application of a constant-voltage input to the second stage. Power drain from a small battery occurs only on switched command of either the siren or horn tones.

Accordingly, it is one of the objects of the present invention to provide novel and improved electronic sound-generating equipment which is advantageously embodied in highly compact solid-state form involving but few inexpensive noncritical components as a result of unique periodicity-modulated pulsing directly through the armature of a sound producer.

Further, it is an object to provide an electronic generator of harmonically-rich tones in which but three low-cost transistors and a speaker having its voice coil directly pulsed by the currents flowing through one of the transistors comprise principal elements of an improved network capable of developing slowly-modulated audio signals.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, which is in schematic form, an improvedcombination siren-horn network embodying the invention is shown to include first and second transistor circuit stage, 2 and 3, respectively, which are selectably energized via a power source 4, such as that afforded by a conventional 9-volt battery 4a. Depending upon the mode of operation commanded by closing a ganged pair of single-pole single-throw switches, 5, or by closing a simple single-throw switch, 6, shown as a pushbutton switch, the network may be commanded to excite an associated loudspeaaker 7 to emit harmonically rich audio tones which automatically ascend and descend in frequency after the fashion of a siren, or, instead, which remain substantially fixed as a constant tone or hornlike sound.

Main stage 3 of the network advantageously includes two relatively inexpensive germanium transistors, 8 and 9, which may for example comprise the Types GA3292 and GP2237, respectively, sold by Texas Instruments, Inc. Voltage level at the base of input transistor 8 is affected by the voltage-dividing resistance 10 (example: in association with resistances 11 (150 k) and 12 (560 k), which are serially connected across the source 4 when either switch 5 or 6 is closed. The emitter of transistor 8 is grounded at the negative supply potential, while its collector is connected to the base of the second transistor, 9: the emitter of the latter transistor is connected to the positive side of the supply, and its collector is grounded by way of a loading impedance which is that posed by the voice coil 7a of the loudspeaker 7. importantly a conventional low-impedance (such as 8 ohms) voice coil provides a highly satisfactory direct loading match in the illustrated circuit relationship, and need for the expected output transformer is obviated. Transistors 8 and 9 are arranged as inverting amplifiers, and the capacitor 13 coupling the collector of the second transistor, 9, with the base of the first, 8, occasions the positive feedback which gives rise to desired oscillatory effects in this stage. Capacitor 13 is typically a common 0101 mfd. capacitance for purposes of promoting oscillatory effects in the range of about 200-800 cycles, and, in association with the two' inverting transistors, effectuates the needed phase shift which achieves regeneration. The currents witnessed by voice coil 7a are in pulseTorm. Resulting sonic outputs from speaker 7 are thus well enriched with harmonics, and produce agreeable audio tones rather than harsh noise.

When switch 6 is first closed, applying a positive voltage to the emitter of transistor 9, the base of transistor 8 is first maintained effectively at the circuit ground or battery-negative potential through the low impedance of speaker coil 7a. Capacitor 13, which initially has zero voltage across it, then commences charging and raising the voltage at the base of transistor 8, the charging rate being determined by the values of the impedances of speaker voice coil 7a and resistances l0 and 11, and by the voltage at the junction of resistances 11 and 12. Capacitor l3 continues to charge until the potential at the base of transistor 8 causes that transistor to conduct, whereupon the potential at the collector of transistor 8 and at the base of transistor 9 is reduced and causes transistor 9 to start to 'turn on. The resulting positive-going potential at the collector of transistor 9 is A-C coupled to the base of transistor 8 through capacitor 13, and this positive-feedback action continues rapidly until both transistors are on hard." At that time, the collector of transistor 9 is clamped at essentially the full battery potential level, and capacitor l3 charges through the low base to emitter saturation resistance of transistor 8, with the result that the potential at the base of transistor 8 begins to fall. Ultimately, the latter potential becomes low enough to start turnoff of transistor 8, whereupon the potential at the collector of transistor 8 and the base of transistor 9 increases and starts turnoff of transistor 9. A negative-going potential then appears at the collector of transistor 9, and is A-C coupled through capacitor 13 to the base of transistor 8, such that both transistors are ultimately full off. The negative; potential resulting at the base of transistor 8 then begins to change positively, at a rate determined by the impedances of voice coil 70, capacitor 13, and resistances l and 1 l, and by the voltage at the junction of 11 and 12, whereupon the cycle repeats.

lmportantly, the potential caused to exist at the junction of resistances 11 and 12 has a controlling effect upon the charging rate of capacitor 13, and the latter in turn controls the frequency of the operation of stage 3. As that potential is raised, the charging rate increases and the signals developed through speaker 7 are of higher pitch. Stage 2 varies that potential for purposes of achieving siren tones.

In stage 3, the uses of NPN transistor 8 and PNP transistor 9 promote the double inversion which is needed to establish the positive feedback condition through capacitor 13. This has the added advantage of providing two stages of voltage and power amplification. Transistor 8 may therefore be a relatively lowpower unit, while the power rating of transistor 9 is high enough to permit direct coupling of the speaker voice coil to its collector. By way of distinction, an astable multivibrator arrangement including collector loads of the usual same value, would result in higher power losses than with the improved circuitry. The latter is thus more efficient and insures longer battery life.

The effective charge-discharge rate characteristics for feedback capacitor 13 are a function of the voltage appearing at the junction of 11 and 12. So long as this voltage is substantially constant, as it is when pushbutton switch 6 alone is depressed, these characteristics are substantially constant, and the dominant audio tone developed by speaker 7 is essentially fixed; it serves admirably as a horn" tone, for example. There are more commanding attention-getting qualities in a variabletone output, such as the ascending and descending fluctuation of a siren, and, for certain amusement, accessory and alarm application, these fluctuations should be readily produced. Network stage 2 serves such purposes, by delivering a substantially sinusoidal output and tone-control voltage to the aforesaid junction. Upon closure actuation of the enabling ganged sections of switch 5, the frequency-modulating or controlling stage 2 is likewise energized by source 4, such that its single common inexpensive transistor 14 (such as the Texas Instruments, Inc. germanium transistor Type GA3292 then has the source voltage applied across the series combination of the load resistance 15 (example: l0 k) and its collector and grounded emitter. Stage 2 comprises a form of phase-shift oscillator, in which resistor 16 biases the transistor 14 on, and the phase-shifting ladder network of capacitors l7a-l7c (example: each about 30 mfd.) and resistances 1811-180 (example: each about 30 k) introduces a phase-shift of about 180 between the base and collector of transistor 14 at a predetermined very low frequency, determined by the component valves, which is significantly less than the repetition rate of the pulsing effects in stage 3. The substantially sinusoidal oscillations generated in stage 2 are characterized by related low frequency sinusoidal variation in voltage appearing at the collector of transistor 14, and, therefore, at the switching terminal 5a and junction of resistances 11 and 12. The same variations in voltage in turn vary the charging and discharging characteristics of capacitor 13, in a manner described hereinabove, with the result that the repetition rate of the pulsing through transistor 9 and speaker 7 is cyclically increased and decreased rather slowly. Sonic outputs from speaker 7, in this mode of operation, are sirenlike and serve well as alarm signals.

Operation of the network is essentially instantaneous when switch closures take place, and there is no standby power drain, and there is no dissipation of power in the low frequency modulating stage 2 unless the undulating output tones are intended. in the case of alarms which are to be triggered by accessory devices, including such items as fuses, relays, and so forth, the illustrated manually operated switches are of course replaced by their automatic counterparts. Some applications may not require a fixed-tone output, in which case the related switching is not necessary, and, in other instances, several dif ferent fixed tones may obviously be readily produced through use of different switches or keys each associated with a different resistance in control of a different voltagc level at the input of stage 3.

It should be understood that the specific preferred embodi' ment and practices described herein have been presented by way of disclosure rather than limitation, and those skilled in the art will appreciate that various modifications, combinations and substitutions may be effected without departure from the spirit and scope of this invention in its broader aspects and as set forth in the accompanying claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. Electronic tone-producing apparatus comprising a loudspeaker having a voice coil, means producing potential the level of which is related to pitch of tone desired from said speaker, first and second transistors, means interconnecting said transistors in a double inversion amplifying relationship, means directly connecting said voice coil in the current flow path of said second transistor, a capacitance connected to supply positive feedback between the output of said second transistor and the input to said first transistor, and to cause self-sustaining cyclic operations of said transistors, and means applying potential from said potential-producing means to said capacitance in control of the charging and discharging rates thereof, whereby the periodicity of electrical signals through said voice coil and the pitch of tones produced by said speaker are determined by said potential.

2. Electronic tone'producing apparatus as set forth in claim 1 wherein said capacitance is of impedance value and said potential is within levels which cause said transistors to be turned on and off cyclically at audio frequency rates, and wherein said means producing said potential comprises an oscillator producing potentials which vary cyclically at a very low frequency which is below audiofrequency, whereby the pitch of tones produced by said speaker varies at said low frequency.

3. Electronic tone-producing apparatus comprising a loudspeaker having a voice coil, means producing potential the level of which is related to pitch of tone desired from said speaker, a first NPN transistor, a second PNP transistor, means connecting the collector of said first transistor with the base of said second transistor, means directly connecting said speaker voice coil serially with the collector of said second transistor, a capacitance coupling said collector of said second transistor with the base of said first transistor, means applying to the base of said first transistor potential related to the potential from said potential-producing means, and means for applying potential across the series combination of said voice coil and the emitter-collector path of said second transistor, whereby periodic electrical signals are developed through said voice coil and said speaker produces audio tones determined by said potential level.

4. Electronic tone-producing apparatus comprising a loudspeaker having a voice coil, means producing potential the level of which is related to pitch of tone desired from said loudspeaker, a first NPN transistor, a second PNP transistor, means connecting the collector of said first transistor with the base of said second transistor, means directly connecting said loudspeaker coil serially with the collector of said second transistor, a capacitance coupling said collector of said second transistor with the base of said first transistor, means applying to the base of said first transistor potential related to the potential from said potential-producing means, means for applying potential across the series combination of said voice coil and the emitter-collector path of said second transistor, said means for applying potential across said series combination comprising a battery, positive potential from said battery being applied to the emitter of said second transistor and the collector of said second transistor being connected with the negative-potential side of the battery through said voice coil, and means connecting the emitter of said first transistor with substantially sinusoidally at a frequency which is low relative to frequency of said audio tones, and further comprising switching means for selectably connecting said sinusoidally varying potentials to said opposite end of said voltage-dividing resistance, to cause said speaker to develop cyclically varying siren tones, and for selectably connecting said opposite end of said voltage-dividing resistance with the positive side of the battery, to cause said speaker to develop a substantially fixed tone. 

1. Electronic tone-producing apparatus comprising a loudspeaker having a voice coil, means producing potential the level of which is related to pitch of tone desired from said speaker, first and second transistors, means interconnecting said transistors in a double inversion amplifying relationship, means directly connecting said voice coil in the current flow path of said second transistor, a capacitance connected to supply positive feedback between the output of said second transistor and the input to said first transistor, and to cause self-sustaining cyclic operations of said transistors, and means applying potential from said potential-producing means to said capacitance in control of the charging and discharging rates thereof, whereby the periodicity of electrical signals through said voice coil and the pitch of tones produced by said speaker are determined by said potential.
 2. Electronic tone-producing apparatus as set forth in claim 1 wherein said capacitance is of impedance value and said potential is within levels which cause said transistors to be turned on and off cyclically at audio frequency rates, and wherein said means producing said potential comprises an oscillator producing potentials which vary cyclically at a very low frequency which is below audiofrequency, whereby the pitch of tones produced by said speaker varies at said low frequency.
 3. Electronic tone-producing apparatus comprising a loudspeaker having a voice coil, means producing potential the level of which is related to pitch of tone desired from said speaker, a first NPN transistor, a second PNP transistor, means connecting the collector of said First transistor with the base of said second transistor, means directly connecting said speaker voice coil serially with the collector of said second transistor, a capacitance coupling said collector of said second transistor with the base of said first transistor, means applying to the base of said first transistor potential related to the potential from said potential-producing means, and means for applying potential across the series combination of said voice coil and the emitter-collector path of said second transistor, whereby periodic electrical signals are developed through said voice coil and said speaker produces audio tones determined by said potential level.
 4. Electronic tone-producing apparatus comprising a loudspeaker having a voice coil, means producing potential the level of which is related to pitch of tone desired from said loudspeaker, a first NPN transistor, a second PNP transistor, means connecting the collector of said first transistor with the base of said second transistor, means directly connecting said loudspeaker coil serially with the collector of said second transistor, a capacitance coupling said collector of said second transistor with the base of said first transistor, means applying to the base of said first transistor potential related to the potential from said potential-producing means, means for applying potential across the series combination of said voice coil and the emitter-collector path of said second transistor, said means for applying potential across said series combination comprising a battery, positive potential from said battery being applied to the emitter of said second transistor and the collector of said second transistor being connected with the negative-potential side of the battery through said voice coil, and means connecting the emitter of said first transistor with said negative-potential side of the battery, said means applying potential to the base of said first transistor comprising voltage-dividing resistance connected at one end with said negative side of the battery and at the opposite end with said means producing potential and at position in between with said base of said first transistor, whereby periodic electrical signals are developed through said voice coil and said loudspeaker produces audio tones determined by said potential level.
 5. Electronic tone-producing apparatus as set forth in claim 4 wherein said means producing potential comprises a single-transistor phase-shift oscillator producing potentials varying substantially sinusoidally at a frequency which is low relative to frequency of said audio tones, and further comprising switching means for selectably connecting said sinusoidally varying potentials to said opposite end of said voltage-dividing resistance, to cause said speaker to develop cyclically varying siren tones, and for selectably connecting said opposite end of said voltage-dividing resistance with the positive side of the battery, to cause said speaker to develop a substantially fixed tone. 